In　this　study,　experiments　on　the　vented　deflagrations　of　hydrogen-methane-air　mixtures　were　performed　in　a　duct　of　length　(300　mm)　x　width　(300　mm)　x　height　(1000　mm)　duct　possessing　a　250　mm　x　250　mm　vent　at　the　top,　to　investigate　the　effect　of　equivalence　ratio　(phi),　in　the　range　of　0.6-1.8,　on　flame　evolution　and　pressure　build-up　within　and　outside　the　duct　at　an　initial　pressure　of　101　kPa　and　an　initial　temperature　of　285　K.　phi　significantly　affects　the　pressure-time　histories　and　flame　propagation　in　the　duct.　Helmholtz　oscillations　of　the　lower　flame　front,　which　resulted　in　violent　Taylor　instability,　were　observed　in　all　the　tests.　Acoustic　oscillations　occurred　at　phi　＞=　1.0,　and　three　pressure　peaks　could　be　distinguished.　The　pressure　peak　which　dominated　the　pressure-time　histories　depended　on　phi　and　the　distance　to　the　vent.　At　a　given　phi,　the　maximum　internal　overpressure　increased　with　the　distance　to　the　vent.　The　maximum　internal　and　external　overpressures　first　increased　as　phi　increased　from　0.6　to　1.0.　Thereafter,　they　decreased　with　further　increase　in　phi.